In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. Under the miniaturizing trend, the lithography has achieved formation of finer patterns by using a light source with a shorter wavelength and by a choice of a proper resist composition for the shorter wavelength. Predominant among others are positive resist compositions which are used as a single layer. These single layer positive resist compositions are based on resins possessing a structure having resistance to etching with chlorine or fluorine gas plasma and provided with such a resist mechanism that exposed areas become dissolvable. Typically, the resist composition is coated on a patternable substrate and exposed to a pattern of light, after which the exposed areas of the resist coating are dissolved to form a pattern. Then, the patternable substrate can be processed by etching with the remaining resist pattern serving as an etching mask.
In an attempt to achieve a finer feature size, i.e., to reduce the pattern width with the thickness of a resist film kept unchanged, the resist film becomes low in resolution performance. If the resist film is developed with a liquid developer to form a pattern, the so-called “aspect ratio” (depth/width) of the resist pattern becomes too high, resulting in pattern collapse. For this reason, the miniaturization is accompanied by a thickness reduction of the resist film (thinner film). On the other hand, with the progress of the exposure wavelength toward a shorter wavelength, the resin in resist compositions is required to have less light absorption at the exposure wavelength. In response to changes from i-line to KrF and to ArF, the resin has made a transition from novolac resins to polyhydroxystyrene and to acrylic resins. Actually, the etching rate under the above-indicated etching conditions has been accelerated. This suggests the inevitableness that a patternable substrate is etched through a thinner resist film having weaker etching resistance. It is urgently required to endow the resist film with etching resistance.
Meanwhile, a process known as multilayer resist process was developed in the art for processing a patternable substrate by etching. The process uses a resist film which has weak etching resistance under the etching conditions for the substrate, but is capable of forming a finer pattern, and an intermediate film which has resistance to etching for processing the substrate and can be patterned under the conditions to which the resist film is resistant. Once the resist pattern is transferred to the intermediate film, the substrate is processed by etching through the pattern-transferred intermediate film as an etching mask. A typical process uses a silicon-containing resin as the resist composition and an aromatic resin as the intermediate film. In this process, after a pattern is formed in the silicon-containing resin, oxygen-reactive ion etching is carried out. Then the silicon-containing resin is converted to silicon oxide having high resistance to oxygen plasma etching, and at the same time, the aromatic resin is readily etched away where the etching mask of silicon oxide is absent, whereby the pattern of the silicon-containing resin is transferred to the aromatic resin layer. Unlike the single layer resist film, the aromatic resin need not have light transmittance at all, allowing for use of a wide variety of aromatic resins having high resistance to etching with fluorine or chlorine gas plasma. Using the aromatic resin as the etching mask, the patternable substrate can be etched with fluorine or chlorine gas plasma.
Typical resins used in the bilayer resist process are polysilsesquioxanes (SSQ). In chemically amplified resist compositions of negative type, SSQ having side chains exhibiting solubility in alkaline developer is typically used in combination with crosslinkers and photoacid generators. In chemically amplified resist compositions of positive type, SSQ having acidic side chains protected with acid labile groups is typically used in combination with photoacid generators. In general, the SSQs are prepared through condensation reaction of silane monomers having three silicon-bonded hydrolyzable groups. Since an ordinary synthesis method yields a SSQ product with a noticeable amount of silanol groups left therein, compositions containing the same suffer from a shelf stability problem due to the instability of silanol groups.
Cage silsesquioxanes are typical of silsesquioxanes (SSQ) which are substantially free of silanol groups, that is, have a degree of condensation of substantially 100%. In general, polyhedral oligomeric silsesquioxanes of 6 to 12 monomer units are known and abbreviated as POSS, of which an oligomer of 8 monomer units (referred to as octet) is relatively readily available. Patternable material using POSS compound is described in US Patent Application 2004-0137241 A1 (corresponding to JP-A 2004-212983, simply referred to as Reference 1). A patternable composition is synthesized by starting with a POSS compound having a dimethylsilyloxy group as a spacer substituted on the POSS skeleton, and effecting hydrosilylation utilizing a bond between silicon and hydrogen for introducing functional side chains therein.
While a patternable material using a POSS compound is described in Reference 1, the disclosed method, that is, the method of introducing a functional group into the POSS compound encounters the problem that an attempt to introduce sterically bulky substituent groups as the functional group using a short spacer would fail to introduce them at entire positions available for substitution. Although it is suggested to derive POSS compounds from SSQ compounds having functional groups, how to implement is not disclosed at all. This technique relates to a patternable composition for forming a low dielectric constant layer. In an attempt to formulate the same as a resist composition having higher resolution, it is difficult to form a microscopic structure for the reason that the length of acid diffusion during pattern formation is increased if a film-forming material has a low glass transition temperature (Tg). Then, in order for this composition to serve as a resist composition with high resolution, it must have a Tg above a certain level. It is preferred to this end that all substituent groups be bulky substituent groups with a short spacer, and possibly without a spacer.
As discussed above, SSQ having a degree of condensation of substantially 100% can have an elevated Tg by introducing side chains of sterically bulky structure so that it becomes an appropriate material for forming a microscopic structure by lithography. In an attempt to introduce bulky side chains into a compound having POSS skeleton by the method disclosed in Reference 1, however, the rate of introduction is restricted.
A list of references includes JP-A 212983 (Reference 1), JP-A 2004-359669, JP-A 5-97719, JP-A 2000-506183, JP-A 2002-55456, JP-A 2004-354417, and Journal of Photopolymer Science and Technology, 18, 631-639, 2005.